In duplicators (or copiers), printers, etc. which utilize an electrophotographic technique, various toners comprising coloring agents and fixing resin have been used to develop electrostatic latent images formed on a photoreceptor having a photosensitive layer containing an inorganic or organic photoconductive material.
Chargeability of such toner is an important factor in a system for developing the electrostatic latent images. Accordingly, in order to appropriately control or stabilize the charge amount of the toner, a charge control agent having a positive or negative charge-granting property has often added to the toner.
As a conventional charge control agent that has been practically used and grants a negative charge to the toner, metal complex salt dyes of monoazo compounds, metal salts or metal complexes of aromatic hydroxycarboxylic acids such as alkyl salicylic acid, etc., can be exemplified. Among them, many of the metal complexes having a structure of azo dyes which have been used as charge control agents have generally a poor stability. For example, they could be decomposed or deteriorated so that their charge control characteristics could be easily impaired due to mechanical friction or shock, electrical impact, exposure to light, or changes in temperature or humidity conditions. In addition, even if charge control agents have a practical level in terms of charge granting property, many of those agents cannot be used as the charge control agents for color toner because they have unsatisfying charge stability or they have a color.
The conventional charge control agents which are colorless or have a white color or light color can be applicable to color toner in terms of color, but almost all agents has problems in terms of performance. They have such disadvantages that high light uniformity has not yet achieved and there is a change in density of images in the durability test. Further, some charge control agents have such drawbacks as difficulty in taking a balance between image densities and fogging, difficulty in obtaining a sufficient image density in a high humidity environment, poor dispersibility to the resin, adverse effects on the storage stability and fixability, etc.
As a charge control agent that applies (grants) a negative charge and can contribute to further improve the performance of copiers, printers, etc., in which color toner is used, for example, calix n arene (a cyclic condensates obtained from an aldehyde and phenol) (Patent Document 1), metal complexes of resorcinol n arene or calix n arene (Patent Document 2), specific dimeric condensates obtained from an aldehyde and a phenol (Patent Document 3), phenol derivative compounds which contain no heavy metals (Patent Document 4), etc. have been known so far.
These cyclic condensation products have high melting points, are strong due to its cyclic structure and have very low solubility in organic solvents. Therefore, when they are added into the toner as the charge control agents, a high charge amount can be achieved, but it is difficult to disperse them in the toner. In particular, in the case of using a low viscosity resin for the color toner, dispersion becomes difficult, and toner scattering is liable to become deteriorated. Further, more efficient electron transfer is essential for improving the charge amount.
To improve the electron transferring, it is advantageous to increase an electronic bias by substituting the para position of the phenyl group, which is an opposite position with respect to the phenolic hydroxyl group (an electron-donating group), with an electron-withdrawing group. However, in cyclic compounds, it is not easy to synthesize electron-withdrawing group-substituted compounds. Accordingly, compounds which are other than the cyclic compounds such as calix n arene, resorcinol n arene, etc., and which have a favorable charging characteristics equivalent to the same effects as that of the cyclic compounds, have been desired.
In order to solve these problems, modification of chain compounds has been considered to be more practical.